This invention relates to a method of assembling electronic packages and, more particularly, to a method of assembling electronic packages for surface mount applications.
As the demand for more efficient manufacturing methods and denser packaging of electronic products grows, more and more manufacturers are turning to mounting the required components in what is known as surface mount technology, rather than using traditional leaded components fed through holes in a printed circuit board and wave soldered on the bottom of the board. In surface mount technology processing, solder pastes are screened on a printed circuit board (generally referred to as a master board since the components being soldered may themselves include printed circuit boards), components are placed with their leads in the paste and the entire master board is brought to the melting temperature of the solder paste to thereby solder all the components to the board.
For discreet components such as integrated circuits, capacitors, resistors etc., the soldering process and the high temperatures involved are not destructive. However, for packaged sub-circuits, such as delay lines, filters or other assemblies that consist of a multitude of individual components, the soldering temperature used to reflow the pastes on the master board may reflow pastes used in the packaged assembly causing internal shorts or open circuits. The technique used to overcome this problem is to use solder pastes in the packaged circuit that have a higher melting temperature than the temperature used to reflow the solder pastes on the master board. This technique is reasonably successful. However, it has been found that because of the differences of thermal expansion of the encapsulating materials used to package the sub-circuits and the components and solder joints within the sub-circuit, tremendous pressures are exerted on encapsulated components and solder joints. These pressures can be great enough to flatten out or extrude internal solder joints causing shorts or opens.